This invention relates to high voltage components, such as a high voltage stator coil, and more particularly, to a high voltage component having a covering for relieving the build-up of internal voltage, thereby preventing partial discharge.
Many high voltage components, such as a stator coil, use an outer conductive ground electrode to ensure that the outer surface is at ground potential. In some of these applications, the ground electrode extends several inches beyond the end of the typically iron stator core into the coil""s end turn region, i.e., the involute region. On high voltage stator bars that have the ground plane extended into the coil involute region, in the region where the ground plane extends beyond the stator core, partial discharge may occur because the involute ground plane may be exposed to physical damage, including mechanical rubs, abrasive particles, surface contamination, and higher than normal voltage stress and temperatures. The end of the core is also a region where mechanical rubs could cause the coil surface to make and break ground contact with the core. With any mechanical rub from the core edge, a support block or a band rub causes localized damage in the surface of the outer ground electrode. When a make and break contact occurs with the outer surface ground electrode and core, current interruption will occur and cause localized partial discharges. When these partial discharges are allowed to occur, surface discharge erosion can occur in the coil""s outer ground electrode. If the erosion in the outer ground electrode is allowed to progress, extensive failure of the involute outer ground electrode may occur. If partial discharge activity is allowed to continue over time, it is possible for complete coil ground wall failure to occur in this region.
Some prior art teachings have provided an improved wall structure for insulating an exterior surface of a high voltage component, such as a stator coil. Although the structures provide some mechanical support and covering, many prior art solutions have not provided protection for partial discharge activity, and only minimal protection for physical damage, including mechanical rubs, abrasive particles, surface contaminations, and higher than normal voltage stress and temperatures. Many prior art techniques have not provided adequate protection for the end of the core and coil, which is a region where mechanical rubs can cause the coil surface to make and break ground contact with the core.
One prior art technique is disclosed in U.S. Pat. No. 5,175,396 to Emery, et al. the disclosure which is hereby incorporated by reference in its entirety. In Emery et al., an improved wall structure insulates the exterior surface of a high voltage component, such as a high voltage coil having roebeled windings. An inner layer is formed from an insulating material disposed over the surface of the component. An outer ground wall grounds the wall structure. A layer of semiconductive material reduces the electric stress across the inner insulating layer and is preferably disposed between the outer surface of the inner insulating layer and the inner surface of the ground wall. A connection can be provided between the high voltage component and the layer of semiconductive material to further lower the electric stress over the inner insulating layer.
In U.S. Pat. No. 6,043,582 to Markovitz et al., a high voltage armature bar has one or more tiers, i.e., columns of conductor strands. Strand insulation surrounds each of the conductor strands. A conductive internal grading is positioned on and surrounds the tier of conductor strands. A ground wall insulation surrounds the conductive internal grading. A conductive material surrounds the ground wall insulation. A transposition filler can be formed by a molding compound at the opposite ends of the copper strand tiers. This molding compound could be formed of a conductive material.
Although these prior art teachings provide some insulator benefit and protection, it is desirable to provide greater protection for high voltage stator bars and the like.
The present invention is advantageous and provides a structure and method to prevent partial discharge in the conductive electrode and coil involute regions of high voltage air cooled stator coils.
In accordance with one aspect of the present invention, a high voltage component is formed as a stator coil, and includes a conducting coil and a ground wall insulation layer overlying and surrounding the conducting coil. An inner grounding electrode overlies and surrounds at least a portion of the ground wall insulation layer. An insulator overlies and surrounds at least a portion of the inner grounding electrode. An outer grounding electrode overlies and surrounds at least a portion of the insulator and is electrically connected to the inner grounding electrode in at least one location for relieving a build-up of internal voltage, thereby preventing partial discharge.
In still another aspect of the present invention, a high voltage stator used in power generation systems includes a core section having a core and at least one stator coil, including at least one coil involute. A ground wall insulation layer overlies and surrounds the at least one stator coil. An inner grounding electrode overlies and surrounds at least a portion of the ground wall insulation layer, including the at least one coil involute. An insulator overlies and surrounds the inner grounding electrode and leaves at least one exposed area of the inner grounding electrode. An outer grounding electrode overlies and surrounds the insulator and is electrically connected to the inner grounding electrode at the at least one exposed area of the inner grounding electrode for relieving a build-up of internal voltage, thereby preventing partial discharge.
The inner grounding electrode is formed from a conductive tape. The core has a core end and the conductive tape forming the inner grounding electrode has a start point that is in board from the core end. The coil involute includes an outboard end and a voltage grading tape is positioned on the outboard end and has a start point. The conductive tape forming the inner grounding electrode is applied beyond the start point of the voltage grading tape. The insulator forming the insulating tape covers the conductive tape forming the inner grounding electrode and leaves the exposed ends of the conductive tape forming the inner grounding electrodes. The outer grounding electrode can include a conductive tape that contacts the exposed ends of the conductive tape forming the inner grounding electrode. This conductive tape forms the inner grounding electrode and has a resistance of about 400 to about 1,000 ohms/square. The insulator comprises an insulating tape and can be a glass binder tape.
In still another aspect of the present invention, a method prevents partial discharge in a high voltage stator coil used in a power generation system. This method includes the steps of surrounding a high voltage stator coil with a ground wall insulation and applying an inner conductive electrode over the ground wall insulation. An insulator is applied over the inner conductive electrode. An outer conductive electrode is applied over the insulator while contacting the inner conductive electrode with the outer conductive electrode, thereby decreasing potential and preventing partial discharge. The inner conductive electrode, insulator and outer conductive electrode can be formed from a tape material.